1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for boosting low- and/or high-frequency content for seismic sources.
2. Discussion of the Background
Reflection seismology is a method of geophysical exploration to image the subsurface of the earth for determining its properties, which information is especially helpful in the oil and gas industry. Typically a controlled source sends seismic energy waves into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
For land applications, vibratory sources are commonly used. Vibratory sources, including hydraulically powered sources, electro-dynamic and sources employing piezoelectric or magnetostrictive material, can generate signals that include various frequency bands, commonly referred to as “frequency sweeps.” In other words, the frequency band of such sources may be controlled.
Seismic vibrators in use today have constraints that impose frequency-variant limits on their output amplitude spectrum. Certain constraints have been recognized in the art. For instance, Bagaini et al. (U.S. Pat. No. 7,327,633, the entire disclosure of which is incorporated herein by reference), have recognized that mass displacement (or “stroke”) of a seismic vibrator device imposes a constraint on the frequency content emitted by the vibratory source. However, while a given constraint, such as a mass displacement, of a seismic vibrator has been considered when designing a sweep for achieving a desired target output spectrum by the seismic vibrator, such consideration of a single constraint fails to take into account other constraints that may impose limitations on the sweep, and thus the resulting designed sweep may fail to operate properly when implemented on the seismic vibrator.
Thus, Sallas (patent application Ser. No. 12/576,804, herein '804, the entire content of which is incorporated herein by reference), explores and takes into account various constraints (not only the stroke limitation) that impose frequency-variant limits on the source output amplitude spectrum. These constraints include but are not limited to: reaction mass stroke, maximum deliverable pump flow, holddown weight, servo-valve response, available supply pressure, and driven structure response. The problem is compounded by other effects like absorption of high frequency energy and environmental noise. While a conventional linear sweep may work well enough to image the subsurface given enough sweep time, it may not provide the most economical solution especially if it requires the use of very long sweep times or many shots at a particular location.
Thus, '804 disclosed a sweep generator that employs a procedure that creates a nonlinear swept sine wave sweep to build up the sweep spectral density to achieve a target spectrum (that is defined by the user to meet the geophysical survey objectives) in compliance with (i.e., without violating) various constraints of the seismic vibrator. '804 also considers other constraints such as environmental constraints (which may be defined by an operator or derived from prior data about a target location), and the disclosed sweep generator employs a procedure for determining a sweep (e.g., a nonlinear sweep) to achieve the target spectrum in compliance with those other constraints in addition to or instead of the constraint(s) of the seismic vibrator that are accounted for by the sweep generator.
For example, '804 discloses that when working near populated areas it may be desirable to reduce the instantaneous peak amplitude of the vibrator force through a certain range of frequencies so as not to excite some structural resonance. Likewise, the sweep generation techniques described in '804 may be implemented to compensate for a drop in instantaneous amplitude through a range of frequencies imposed by environmental constraints and a suitable nonlinear sweep may be generated to build up the sweep spectral density to achieve a target spectrum.
However, the aforementioned techniques for generating sweeps that compensate for system constraints that fall in the low- and high-frequency range are designed for use with swept sine wave excitation signals and are not well suited for use with pseudo-random excitation signals. Existing techniques that are designed for use with swept sine waves include compensation methods for the low-frequency end that avoid the possibility of driving the source to reach the stroke limitations, i.e., the reaction mass of the source may reach the stops. At the high-frequency end, especially if it is desired to mimic the spectrum of a non-linear sweep designed to overcome the high-frequency attenuation of the earth (absorption), the existing techniques reduce the risk of overdriving the servo-valve or even run into overpressure situations in the actuator that can lead to working fluid cavitation.
With interest in using unconventional sweeps to increase productivity through use of separable simultaneous sources, there is a need for a corresponding pilot sweep design method that is suitable for use with pseudo-random pilot sweeps that maximizes the energy in the sweep while still honoring the target spectrum without exceeding system constraints: that is, a pilot signal configured to drive a seismic vibratory source to avoid the stroke limitations at the low-frequency end and to not overdrive the servo-valve of the source at the high-frequency end and, at the same time, to boost the low- and high-frequency ends (content) as these parts of the spectrum are important for imaging the subsurface.
With swept sine wave sweeps the frequencies usually change monotonically and at any point in time there is only one frequency or a very narrowband range of frequencies in the pilot signal. Pseudo-random pilot sweeps impose special problems because at any point in time a plurality of frequencies is present simultaneously and their subsequent impact on system demand more difficult to predict. Furthermore, it is desirable for the output of the vibrator to follow the pilot signal, since typically the pilot signal is used as a correlation operator and assumed to be representative of the emitted energy. The drive level for the source radiated output is chosen so that the vibrator does not exceed some system limit within its sweep. Drive level is usually defined as a percentile of the holddown weight or the peak force output rating of the vibrator, where for example a drive level setting of 80% would imply that the peak force output of the truck is 80% of the static holddown force applied to the baseplate to keep it in contact with the earth. A reduction in the drive level setting will reduce the force output of the truck throughout the entire sweep time. Reduction in output is undesirable because it will reduce the signal to ambient noise ratio in the received signal.
If for some reason a pseudo-random pilot signal creates peak demands that exceed system limits at any time within the sweep interval, then the vibrator drive level setting will need to be reduced resulting in a reduction in the total emitted energy. So it is desired that the pseudo-random signal be designed to maximize its energy content without creating peak demands that lead to exceed system constraints so that drive level settings can be maximized. Accordingly, it would be desirable to provide systems and methods that overcome the afore-described problems and drawbacks.